In-situ sensors and methods for monitoring environment assisted cracking of structural components

ABSTRACT

Sensor assemblies, units and methods are provided to determine crack development of components of interest associated with a monitored structure. According to preferred embodiments, a sample sensor bolt is provided having a shank with a threaded end, the sensor bolt being formed of a material serving as a surrogate of the material forming a component of interest associated with the monitored structure. A frame surrounds the shank of the sensor bolt and has fluid ports therein to allow fluid to contact an exposed portion of the sensor bolt shank in registry therewith. A load cell is operatively connected to the sensor bolt. A pre-load nut is threaded onto the threaded end of the sensor bolt shank and contacts an end of the frame so as to place the sensor bolt under an initial tensile stress. Crack formation within the sensor bolt shank caused by fluid acting upon the exposed portion thereof responsively relieves the initial tensile stress of the sensor bolt which is thereby sensed by the load cell, whereby crack formation in the shank can be used to sense the propensity for crack development in the component of interest associated with the monitored structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority benefits under 35 USC§119(e) from U.S. Provisional Application Ser. No. 61/272,769 filed onOct. 30, 2009, the entire content of which is expressly incorporatedhereinto by reference.

TECHNICAL FIELD

The technology described herein relates generally to the technical fieldof physical sensors. More specifically, the technology described hereinis in the technical field of structural health monitoring sensors.

BACKGROUND

Environment assisted cracking (EAC) has long been recognized as a majorcause of component and structural failures, but the basic mechanisms ofthis process are still not fully understood. EAC includes hydrogenembrittlement (HE) and stress corrosion cracking (SCC) and other formsof metal and alloy cracking caused by the combined action of stress andenvironment. The development of standardized and practicable methods oftesting and monitoring these degradation mechanisms is critical tounderstanding the kinetics of such failures and also to providemaintainers with an indication of operating conditions that have thepotential to produce EAC in their systems.

One proposed mechanism of EAC is hydrogen embrittlement (HE), which isthe process by which various metals, such as high-strength alloys, loseductility and crack due to exposure to hydrogen. The process isassociated with hydrogen uptake in the alloy, which can be initiated byatomic hydrogen produced by electrochemical processes. The hydrogenatoms affect the ductility of the alloy and promote the propagation oftransgranular and intergranular cracks with reduced overall elongationto failure. The combination of atomic hydrogen, alloy properties, andapplied stress can result in embrittlement and crack propagation. A widerange of metals and alloys are susceptible to HE, includinghigh-strength low-alloy steels and nickel based materials.

Hydrogen embrittlement can occur during various manufacturing operationsor operational uses, i.e. anywhere that the metal comes into contactwith atomic or molecular hydrogen. Processes that can lead to HE includecathodic protection, phosphating, pickling, and electroplating. Othermechanisms of hydrogen introduction into metal are corrosion, chemicalreactions of metal with acids, or with other chemicals, notably hydrogensulfide in sulfide stress cracking, a process of importance for the oiland gas industries.

There are two ASTM standards for testing embrittlement due to hydrogengas. The standard ASTM F1459-06 Standard Test Method for Determinationof the Susceptibility of Metallic Materials to Hydrogen GasEmbrittlement (HE) Test¹ uses a diaphragm loaded with differentialpressure. The test ASTM G142-98 (2004) Standard Test Method forDetermination of Susceptibility of Metals to Embrittlement in HydrogenContaining Environments at High Pressure, High Pressure, HighTemperature, or Both uses a cylindrical tensile specimen tested in anenclosure pressurized with hydrogen or helium. ¹ This and all otherreferenced publications below are expressly incorporated by referenceherein.

Another ASTM standard exists for quantitatively testing for the HydrogenEmbrittlement threshold stress for the onset of Hydrogen-InducedCracking due to platings and coatings from Internal HydrogenEmbrittlement (IHM) and Environmental Hydrogen Embrittlement (EHE)[1]—ASTM F1624-06 Standard Test Method for Measurement of HydrogenEmbrittlement Threshold in Steel by the Incremental Step LoadingTechnique.

The main interest of the environment assisted cracking detection systemis for monitoring high-strength fasteners and structures that may besusceptible to HE under non-ideal cathodic protection conditions.Available hydrogen, in conjunction with high tensile loads and localstress risers that are characteristic of typical bolting applications(e.g., threads, surface imperfections), can result in EAC and subsequentfastener failure that has significant safety and availabilityimplications for a variety of marine structures such as ships, oil andgas platforms, and pipelines. An in situ EAC sensing device wouldtherefore provide valuable early warning capability, alerting users toenvironmental conditions that are prerequisite to cracking within themonitored structure.

Conventional in situ EAC sensing methods approach the problem in variousways. Of these, the methods involving the sensing of mechanical strainrelief under cracking conditions are of particular interest. As oneexample, U.S. Pat. No. 3,034,340 to Jankowsky et al has proposedfracture specimens from sections of pipe, known as c-rings, to produce alarge tensile stress in a notched region when loaded with aninstrumented bolt. Crack initiation and propagation in the notchedregion tends to relax the initial bolt load, and thus is measureable bymonitoring the load (strain) within the bolt member. Another priorproposal in U.S. Pat. No. 7,387,031 to Perrin et al includes a similarapproach where strain gage instrumented flat strips of metal aredeformed and held in a U-shape. Material loss caused by corrosion,erosion, pitting, and cracking are detrimental to the stiffness of themetal sample, thereby resulting in a deflection that is observed by thestrain gage instrumentation.

In order to reduce the equipment burden, permit field deployment, andreduce costs, it is an objective of the technology described herein toproduce a compact device that can supply the necessary stress to inducesample failure under approximate plane-strain conditions. The samplearrangement chosen for the current design is the circumferential notchedtensile (CNT) geometry. As described in Ibrahim, R. N., et al. “Validityof a new fracture mechanics technique for the determination of thethreshold stress intensity factor for stress corrosion cracking (K_(ISCC))and crack growth rate of engineering materials”, EngineeringFracture Mechanics 75 (2008) 1623-1634, the CNT geometry is the smallestpossible geometry that can produce approximate plane-strain crackloading conditions, within 3% of the ASTM compact tension (CT)specimens. To produce valid plane-strain conditions, the sampledimensions must be sufficient to constrain the plastic zone ahead of thecrack tip. The CNT specimen can be made smaller thanks to its continuouscircumferential notch, which affords a highly constrained plastic zone.The CT specimen, on the other hand, must be much thicker to ensure thatthe plane stress conditions at the free surfaces are small compared tothe plane strain region in the interior of the specimen. For example,acceptable results have been obtained with 9.5 mm and 15 mm CNTspecimens for materials that required CT specimen dimensions up to 80mm.

Application of conventional monitoring and warning systems throughoutthe flooded or wetted spaces of a vessel or other structure would becomplex, expensive, heavy, and vulnerable to damage. There is a currentneed for a simple monitoring system that can be used in the vicinity ofcritical high strength components to indicate the cumulative impact ofconditions that can lead to EAC and premature failure. The problem ofmonitoring for conditions leading to EAC, in particular HE is solvedaccording to the technology disclosed herein by utilizing a small CNTspecimen in conjunction with very stiff sensor construction and highlysensitive strain gage instrumentation to provide high resolution crackdepth measurement on the surrogate sample.

The technology described herein is embodied in novel sensors and methodsfor detecting the presence of conditions that would lead to environmentassisted cracking (EAC) within structural components. According tocertain embodiments, a sensor is provided which contains a materialsample of similar composition to the monitored structure and is placedunder a tensile preload that mimics the loading experienced by themonitored structure. Cracking within this surrogate sample correlates todamage in the monitored structure. The crack depth measurement is madeby comparing the real-time tensile force on the sample to its initialvalue. Cracking in the sample increases its compliance and causes theload to drop in a predictable manner. The sensor design embodied by thetechnology described herein combines a very compact geometry withhigh-resolution crack depth measurement at a low cost, thereby making itvery well suited for field installations, especially for alloys andconditions that have very low crack velocities that would normally goundetected.

Certain embodiments of the invention have the ability to monitor theinitiation and progression of cracks in a tensile specimen with highresolution while in a small, ruggedized package, not requiring a largeload frame or costly instrumentation. Additionally these devices are notsusceptible to changes in fluid conductivity that can confound crackdepth measurement techniques based on sample electrical conductivity.One principal design consideration that permits the high-resolutioncrack depth sensing is the high mechanical stiffness of both the sampleand loading frame that enhances the load drop (sensed parameter) withcrack growth. The sensor device according to embodiments of theinvention is applicable both to real-time condition monitoring of astructure (e.g. underwater pipeline fasteners under cathodic protection)as well as laboratory characterization of the EAC susceptibility ofmaterials, particularly during alloy development when a large number oftests are required for extended periods.

According to preferred embodiments, a sensor unit provides surrogatedetermination of crack development within a component of interestassociated with a monitored structure. The sensor unit most preferablyincludes a sample sensor bolt having a shank with a threaded end. Theshank of the sensor bolt is formed of a material serving as a surrogateof the material forming the component of interest associated with themonitored structure. A frame surrounds the shank of the sensor bolt andhas fluid ports therein to allow fluid to contact an exposed portion ofthe sensor bolt shank in registry therewith. A load cell is operativelyconnected to the sensor bolt. A pre-load nut is threaded onto thethreaded end of the sensor bolt shank and contacting an end of the frameso as to place the sensor bolt under an initial tensile stress. Crackformation within the sensor bolt shank caused by fluid acting upon theexposed portion thereof responsively relieves the initial tensile stressof the sensor bolt which is thereby sensed by the load cell. In such amanner, the crack development occurring in the sensor bolt shank may bedetermined and correlated to crack development occurring within thecomponent of interest associated with the monitored structure.

These and other aspects of the present invention will become more clearafter careful attention is given to the following detailed descriptionof the preferred exemplary embodiments thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to the accompanying drawings wherein likereference numerals through the various FIGURES denote like structuralelements, and wherein

FIG. 1 is a schematic view of an EAC monitoring sensor assembly inaccordance with a preferred embodiment of the present invention in anexemplary environment of use;

FIG. 2 is a perspective view partly in section of the EAC monitoringsensor assembly depicted in FIG. 1;

FIG. 3 is an enlarged side elevational view of a crack sensor unitemployed in the EAC monitoring assembly depicted in FIG. 2;

FIG. 4 is a cross-sectional view of the crack sensor unit shown in FIG.3 taken along lines 4-4 therein;

FIG. 5 is a calibration curve for the EAC monitoring sensor assemblyrelating measured load to nominal crack depth;

FIG. 6 is a plot of the experimental results showing crack depth versustime at various applied potentials;

FIG. 7 is a plot of the experimental results showing crack growth rateas a function of applied electrochemical potential; and

FIG. 8 is a graph of the experimental results showing crack depth versustime at various aerated and deaerated fluid flow rates.

DETAILED DESCRIPTION

The following description sets forth specific details, such asparticular embodiments, procedures, techniques etc. for purposes ofexplanation and not limitation. It will however be appreciated by oneskilled in the art that other embodiments may be employed apart fromthese specific details. In some instances, detailed descriptions of wellknown methods, material and devices are omitted so as not obscure thedescription with unnecessary detail. Individual blocks may be shown insome of the figures.

Accompanying FIG. 1 depicts schematically an EAC monitoring sensorassembly 10 in accordance with one presently preferred embodiment of thepresent invention in an exemplary environment of use. As can be seen,the sensor assembly 10 is positioned so as to be in fluid-communicationwith a fluid 12 which is in contact with a structural componentassociated with the desired monitored structure 14. The fluid 12 may beany environment liquid or gas that contacts the EAC monitoring sensorassembly 10. The assembly 10 includes a protective shroud 10-1 having acircumferentially disposed series of openings 10-2 to allow the fluid 12to flow into the shroud 10-1 and communicate with its internalcomponents to be discussed in greater detail below. Signal dataindicative of the effect that the fluid 12 has on such internalcomponents may be transferred via signal line 15 to a data processor 16or like device. Signal transfer via signal line 15 may be accomplishedby wired or wireless communications. Similarly, to ensure that thesensor components of the assembly and the monitored structure 14 are atthe same electrochemical potential, an electrical connection may beestablished therebetween via line 14-1. The processor 16 thus presents ahuman-readable form of the data to show, for example, the extent ofenvironment assisted cracking that may be occurring to the monitoredstructure by virtue of its contact with the fluid 12.

The EAC monitoring sensor assembly 10 and its associated crack sensorunit 20 are shown in greater detail in accompanying FIGS. 2-4. As isshown, the shroud 10-1 of assembly 10 defines an interior space 10-3containing the crack sensor unit 20, forward and rearward structuralsupports 22, 24 and the electronics package 30 containing the signalcondition electronics and battery (not shown). The electronics package30 is positioned within a fluid-tight chamber 10-4 within the interiorof the shroud 10-1 and is connected to the data processor 16 via signalline 15 as was described previously. A reference electrode 28(preferably a standard Ag/AgCl electrode) is positioned within theinterior space 10-3 of the shroud 10-1 so as to be in contact with thefluid entering the shroud 10-1 via openings 10-2.

The on-board electronics package 30 acquires load cell, temperature, andelectrochemical potential measurements from the respective transducers.The electronics package 30 translates these raw measurements intopractical engineering units (e.g. force, crack depth, crack velocity).The user may stream cracking data in real-time or store the dataon-board in non-volatile (i.e. flash) memory for subsequent download,the latter permitting standalone deployment.

As is shown in FIGS. 3-4, the crack sensor unit 20 includes a generallycylindrical load frame 22 having an electrically insulated coating 22-1thereon. Preferably the insulated coating 22-1 is an epoxy castingapplied over the exterior surface of the frame 22. The frame 22 includesa pair of outwardly protruding engagement end bosses 22-3 a, 22-3 b ateach end thereof. The mechanical stiffness characteristics afforded bythe protruding engagement end bosses 22-3 a, 22-3 b permits the loadframe end faces to deflect in a very uniform manner under load, therebymaintaining a flat interfacing surface for load cell 25 and preload nut26.

A tensile specimen bolt 24 is received coaxially within the frame 22. Itwill be appreciated by those skilled in the art that, given sufficientdimensional tolerancing on the mating components, this embodimentreadily ensures that the sample is not subject to bending momentsresulting from misalignment. The tensile specimen bolt 24 includes acircumferential notch 24-1 formed on a shank 24-2 thereof which isdisposed in registry with the ports 22-2 of the frame 22. In such amanner, therefore, the fluid 12 will contact the specimen bolt 24 at thenotch 24-1. A load cell 25 is sandwiched between end boss 22-3 a of theframe 22 and the head 24-3 of the specimen bolt 24. The terminal end24-2 a of shank 24-2 is threaded and thereby threadably receives apreload nut 26. The nut 26 thus bears against the end boss 22-3 b of theframe 22. As the nut 26 is tightened against the end boss 22-3 b, atensile stress will in turn be responsively exerted on the bolt 24. Themagnitude of the tensile stress will thus be sensed by the load cell 25and be communicated thereby to the electronics package 30. Any change inthe pre-loaded stress indicative of cracking of the bolt 24 in thevicinity of the notch 24-1 thereof will thus be sensed by the load cell25 and communicated via signal line 15 to the data processor 16. Sincethe material of the bolt 24 is a surrogate for components of interestassociated with the monitored structure 14, this sensed cracking of thebolt 24 in the vicinity of the notch 24-1 will thereby be indicative ofenvironment assisted cracking of such components. In such a manner,therefore, the sensor unit 20 serves as a small scale model ofstructural cracking occurring on a larger scale with the components ofinterest associated with the monitored structure 14.

O-ring seals 23 are provided to seal the bolt 24 from the fluid enteringvia ports 22-2 except for the exposed notch 24-1 thereof. The O-ringseals 23 and the insulated coating 22-1 serve to isolate all metalsurfaces except for the notch 24-1 of the specimen bolt 24 from thefluid (e.g. seawater) entering the ports 22-2 once the sensor unit 20 issealed within the overall assembly 10. This sealing arrangement preventsundue EAC within the frame 22 and also avoids galvanic coupling problemsif the frame 22 and sample bolt 24 are formed of disparate materials.For applications in which o-rings would not be desirable from a crevicecorrosion standpoint, alternate sealing arrangements are possible (e.g.RTV sealant).

The tensile specimen bolt 24 is most preferably formed of a surrogatealloy with respect to the monitored structure 14. The preload stressintensity (K_(I)) generated at the root of the notch 24-1 is sufficientto initiate and propagate cracks in the bolt 24 under embrittlingconditions (K_(I)>K_(I, EAC)), but is low enough to delay ductileoverload failure (K_(I)<K_(IC)) until after a usable crack depth rangehas been used. As cracks initiate and grow in the sample bolt 24, itsmechanical compliance increases, thereby relieving some of the initialpreload. The embedded load cell 25 thus continuously monitors thetensile load of the sample bolt 24 and transmits such tensile load datato the data processor 16. A microcontroller associated with the dataprocessor 16 or within the electronics package 30 compares the currentload to the initial load and calculates the current crack depth based oncalibration curves relating load shedding to crack depth. This real-timecrack depth monitoring system may therefore provide vital informationfor three objectives: 1) maintaining proper cathodic protection systemparameters, 2) establishing safe inspection intervals based on potentialcrack growth in a specific environment, and 3) obtaining fractureproperties for new and existing alloys in real-world environments, whichis not possible with traditional test methods requiring bulky andexpensive hardware.

The tensile sample bolt 24 is ideally fabricated from the same alloy asthe components of interest associated with the monitored structure 14,or in the case of laboratory studies, whichever alloy is of interest tothe researcher. The load frame 22 and preload nut 26 may be fabricatedof any material, but preferably using a material with high elasticmodulus and similar thermal expansion properties as the tensile samplebolt 22 to reduce the sensitivity to changes in environmentaltemperature. The load cell 25 according to preferred embodiments can beany unit commercially available and intended for long-term monitoring ofbolt preload and is therefore exceptionally stable over long periods oftime and ideally used for this application. One preferred unit that maybe employed for the load cell 25 is the Model LWO-25 commerciallyavailable from Transducer Techniques, Inc. of Temecula, Calif.

During assembly, the tensile sample bolt 24 is loaded to a predeterminedstress value to produce the desired mode-I stress intensity (K_(I)) inthe root of notch 24-1. Preload stress is applied using a hydraulicfixture that effectively compresses the frame 22 and tensions the samplebolt 24. The preload nut 26 is then tightened by hand to lock in thepreload stress once the hydraulic pressure is removed. Thehydraulic-assisted preload method reduces undesirable torsional stressesin the sample since the required torque on the nut is low compared tothe case without hydraulic assistance where high tightening torques arerequired to achieve the necessary preload. Furthermore, the hydraulicfixture provides a convenient approach for verifying the factory forcecalibration of the units employed as the compression load cell 25.

To ensure that the crack sensor unit 20 is at the same electrochemicalpotential (i.e. propensity for hydrogen embrittlement) as the monitoredstructure 14, an electrical connection is made via line 14-1 between thetensile sample bolt 24 and the monitored structure 14. The cathodicprotection system or anode arrangement (not shown) associated with themonitored structure 14 then polarizes the sample bolt 24 to the samepotential as the exposed areas of the structure 14. In the case of alaboratory test under potentiostatic control, the tensile sample bolt 24becomes the working electrode (WE). Under conditions where the hydrogenevolution reaction is possible, the sample bolt 24 suffers EAC and crackinitiation and growth are monitored.

The embedded electronics package 30 preferably provides several keyfunctions, notably: (1) signal conditioning and digitization of the datafrom load cell 25, embedded temperature sensor, and reference electrode28, (2) crack depth calculation including temperature compensation, (3)long-term data storage, and/or (4) communication with an external device(e.g., the data processor 16 or other display/monitoring systems) inorder to download data from the sensor (e.g. during maintenance cycles).The electrochemical reference potential as measured with the on-boardreference electrode 28 helps corroborate EAC indications provided by thecrack detection load cell transducer 25.

The electronics package 30 may be powered over the serial communicationsbus (e.g. RS485), or for long-term remote testing, powered by batterieshoused within the sensor assembly 10. A hermetically sealed submersibleelectrical connector 40 (see FIG. 1) provides the electrical interfaceto the sensor electronics package 30, permitting fully-submergedoperation without risk of water ingress at the connection.

In use, cracks growing within the sensor sample bolt 24 decreases itsmechanical rigidity, resulting in a drop in the preload stress of thebolt 24 from its initial value that is detected by high resolution forceinstrumentation. Below is a discussion which outlines the analyticalformulation describing the effective load drop with crack growth and howthe force measurement is converted to the more practical crack depthindication via algorithms that may be employed by the electronicspackage 30 or by the data processor 16.

Of the four primary load-bearing components of the crack sensor unit 20,the sample bolt 24 is in tension while the remainder of the componentparts (e.g., the frame 22, nut 26 and load cell 25) are necessarily incompression. If k_(f) is the effective combined stiffness of the loadframe 22, nut 26, and load cell 25, whose value is assumed to beconstant with time, and the tensile sample stiffness, k_(s), is afunction of crack depth (a), then the overall assembly stiffness, k, asthe series-combined k_(f) and k_(s) stiffnesses can be represented bythe following Equation 1:

$\begin{matrix}{{k(a)} = \frac{k_{f}{k_{s}(a)}}{k_{f} + {k_{s}(a)}}} & \left( {{Equation}\mspace{14mu} 1} \right)\end{matrix}$

This expression in turn permits the straightforward expression relatingapplied force, F, to the resulting deflection, x as per Equation 2below:

F(a)=k(a)·x  (Equation 2)

It will be understood that both the sample bolt 24 and load frame 22experience equal force, F, but of opposite sign. Practically speaking,the deflection under load, x₁ may be envisioned as the distance thepreload nut 26 is advanced up the sample bolt threads at its threadedend 24-2 a relative to the unloaded condition. The deflection, x,therefore represents the effective preload strain imparted to theassembly.

Upon initial assembly when the sample is not yet cracked (i.e. a=0), thepreload force F(0) is set to provide the desired stress intensity in theroot of the sample notch 24-1 to ensure cracking under HE conditions.This initial load applied to the effective stiffness with no cracking,k(0) produces a total deflection, x, in the component assembly.Importantly, this total deflection remains constant throughout the lifeof the sensor unit 20. This is a reasonable assumption as there is noreason to expect the total deflection to change unless the nut 26 isloosened or tightened, other than long term creep and thermal strainsthat are discussed subsequently.

If displacement is defined as a constant value, then it isstraightforward to determine the relative change in effective stiffnessby monitoring the relative change in the force, as compared to theinitial states noted by Equation 3:

$\begin{matrix}{\frac{k(a)}{k(0)} = \frac{F(a)}{F(0)}} & \left( {{Equation}\mspace{14mu} 3} \right)\end{matrix}$

From this expression, it is clear that the crack depth may be inferredfrom the present and initial forces if the relationship between tensilesample crack depth and stiffness were known. For example, if themeasured load indicated a 25% reduction in preload force relative to theinitial state, one would simply determine at what crack depth theoverall stiffness had been reduced by the same percentage.

The characteristic sample stiffness vs. crack depth curve is developedby measuring sample deflection at a given load over a range of crackdepths. This study may be performed empirically using a mechanicalloading machine (e.g. Instron, MTS) to characterize the actual samplestiffness for several crack depths, or numerically using finite elementanalysis (FEA) to simulate the same arrangement.

The stiffness vs. crack depth relationship presented in FIG. 5 wasdeveloped using a FEA, where the stiffness has been normalized againstthe initial value, and crack depth normalized against the uncrackedligament diameter, d. The crack is assumed to propagate radially, withuniform depth around the circumference (i.e. concentric). Using Equation1, the sample stiffness (k_(s)), was then corrected for the effects offrame compliance (k_(f)) to produce the assembly stiffness curve (k).The corrected assembly stiffness curve is utilized in the on-boardmicroprocessor to translate measured force (viz. stiffness) into apractical crack depth metric. This curve may be further refined usingempirical measurements of the actual assembly stiffness during thesensor force calibration procedure.

Thermal effects must be mitigated to ensure the best possible crackdepth resolution. Uncompensated, thermal sensitivities are sufficient tocreate significant error in the crack depth calculation over smalltemperature changes (typically 0.01-0.03 mm/° C.). Temperatureinfluences the EAC detection crack sensor unit 20 through two primarymodes. First, the load cell 25 and instrumentation package 30 haveinherent thermal sensitivities that affect the measurement span andoffset, causing an error between the actual and measured applied loads.Second, differential thermal expansion between the frame 22 and samplebolt 24 affect the actual applied force much in the same way asloosening or tightening the preload nut 26—a consequence of theintentionally stiff mechanical assembly. The latter effect can beminimized by using materials to construct the frame 22 and bolt 24 withsimilar thermal expansion characteristics (i.e. the same material). Incases where this is not practical, such as reusing old sensor frames fora new alloy, this effect must be characterized and compensated.

In a preferred sensor embodiment, temperature is measured using athermally sensitive element (e.g. RTD) that may be embedded directlywithin the load frame 22 by recesses 50 shown in FIG. 4. The sensorresponse is characterized over a range of temperature at several appliedloads to characterize the thermal sensitivity. Unloaded without thepreload nut 26, the observed sensitivity is limited to the load cell 25and electronics package 30, as differential strain can produce noadditional load. Once the preload nut 26 is applied, the resultingthermal response is the combined effect of instrumentation anddifferential thermal expansion effects. Given both the unloaded andcombined response, the response to differential thermal expansion alonemay be extracted.

The crack depth calculation is compensated for the effect of temperatureby first correcting the measured force (load cell and instrumentationoutput), then applying a correction factor for the estimated level ofdifferential thermal expansion at the measured temperature.

The present invention will be further understood from the followingnon-limiting experimental testing.

Experimental Test 1

Testing was performed to characterize the sensor's real-time crack depthmeasurement performance. A 17-4PH stainless steel (H900) tensile samplewith an untracked ligament diameter (d) of 10.2 mm [0.400 in] was loadedto a stress intensity of approximately 30 MPa(m^(0.5)), immersed in 0.6M NaCl solution and subjected to a range of applied cathodic potentials.Data was acquired by initiating a crack at −1.25 V relative to astandard calomel electrode (SCE) and stepping the potential to higher(less negative) values at one hour intervals. The measured crack depthtime history is depicted in FIG. 6, and the approximated crack growthrates plotted against the electrochemical potential in FIG. 7.

The linearity of the plot of FIG. 7 suggests that the crack growth rateis dependent on the kinetics of the hydrogen evolution reaction at thecrack tip which is a function of the log of potential. Note that even atpotentials as high as −0.9 V SCE the crack continues to grow. In otherexperiments at lower K_(I) values, it has also been shown that the crackwas completely arrested at potentials greater than about −1.1 V SCE.These findings are consistent with data previously developed by C. T.Fujii, Stress Corrosion Cracking—New Approaches, ASTM STP 610, ASTM,Philadelphia, Pa., pp. 213-225, (1976). For higher values of K_(I), thepotential required for crack growth is less negative.

Experimental Test 2

The real-time crack measurement capability of the EAC crack sensor unit20 has also been demonstrated using Monel K500, a high-strength alloythat is widely employed in marine environments for high stressapplications (e.g. bolting, shafting). While nominally exhibiting hightoughness, this material has proven to be susceptible to the effects ofhydrogen embrittlement resulting in the failure of critical marinestructures.

Direct-aged K500 material (HRc 35) was loaded to an initial stressintensity of 33 MPa(m^(0.5)) and subjected to a cathodic potential of−1.27 V (Ag/AgCl). FIG. 8 illustrates the resulting crack depth timehistory across several operating conditions where the fluid aeration andflow rate were varied in order to ascertain their effect on the crackpropagation velocity. The circles indicate times at which conditionswere changed. It is notable that the sensor responds within minutes tochanging conditions, especially considering the low hydrogen diffusivityof this nickel-based alloy.

1. A sensor unit which provides surrogate determination of crackdevelopment within a component of interest associated with a monitoredstructure comprising: sample sensor bolt having a shank with a threadedend, wherein at least the shank of the sensor bolt is formed of amaterial serving as a surrogate of the material forming the component ofinterest associated with the monitored structure; a frame surroundingthe shank of the sensor bolt and having fluid ports therein to allowfluid to contact an exposed portion of the sensor bolt shank in registrytherewith; a load cell operatively connected to the sensor bolt; and apre-load nut threaded onto the threaded end of the sensor bolt shank andcontacting an end of the frame so as to place the sensor bolt under aninitial tensile stress, wherein crack formation within the sensor boltshank caused by fluid acting upon the exposed portion thereofresponsively relieves the initial tensile stress of the sensor boltwhich is thereby sensed by the load cell, whereby crack development inthe sensor bolt shank may be determined and correlated to crackdevelopment occurring within the component of interest associated withthe monitored structure.
 2. A sensor unit as in claim 1, wherein thesensor bolt shank includes a circumferential notch which is in registrywith the ports of the frame.
 3. A sensor unit as in claim 1, wherein theframe includes an electrically insulative coating.
 4. A sensor unit asin claim 1, wherein the frame includes a thermally sensitive element. 5.A sensor unit as in claim 1, wherein the frame is generally cylindrical,and wherein the sensor bolt is coaxially received by the frame.
 6. Asensor unit as in claim 1, wherein the frame includes protrudingengagement end bosses at each end thereof to produce uniform framedeflection characteristics.
 7. A sensor unit as in claim 7, wherein thesensor bolt includes a head at an end opposite to the threaded end, andwherein the load cell is sandwiched between the head of the sensor boltand an opposed one of the engagement end bosses of the frame.
 8. Asensor unit as in claim 8, wherein the preload nut contacts an oppositeone of the engagement end bosses.
 9. A sensor assembly comprising: ashroud defining an interior space and having an opening therein to allowfluid to enter the interior space from an exterior thereof; anelectrically isolated sensor unit positioned within the interior spaceof the shroud; and an electronics package connected operatively to thesensor unit for generating signal data representative of crack formationdeveloped in the monitored structure, wherein the sensor unit comprises:(i) sample sensor bolt having a shank with a threaded end; (ii) a framesurrounding the shank of the sensor bolt and having fluid ports thereinto allow fluid to contact an exposed portion of the sensor bolt shank inregistry therewith; (iii) a load cell operatively connected to thesensor bolt; and (iv) a pre-load nut threaded onto the threaded end ofthe sensor bolt shank and contacting an end of the frame so as to placethe sensor bolt under an initial tensile stress, wherein crack formationwithin the sensor bolt shank caused by fluid acting upon the exposedportion thereof responsively relieves the initial tensile force of thesensor bolt which is thereby sensed by the load cell.
 10. A sensorassembly as in claim 9, wherein the sensor bolt shank includes acircumferential notch which is in registry with the ports of the frame.11. A sensor assembly as in claim 9, wherein the frame includes anelectrically insulative coating.
 12. A sensor assembly as in claim 9,wherein the frame includes a thermally sensitive element.
 13. A sensorassembly as in claim 9, wherein the frame is generally cylindrical, andwherein the sensor bolt is coaxially received by the frame.
 14. A sensorassembly as in claim 9, wherein the frame includes protruding engagementend bosses at each end thereof.
 15. A sensor assembly as in claim 14,wherein the sensor bolt includes a head at an end opposite to thethreaded end, and wherein the load cell is sandwiched between the headof the sensor bolt and an opposed one of the engagement end bosses ofthe frame.
 16. A sensor assembly as in claim 15, wherein the preload nutcontacts an opposite one of the engagement end bosses.
 17. A sensorassembly as in claim 9, further comprising a data processor operativelyconnected to the electronics package for receiving the signal datagenerated thereby.
 18. A sensor assembly as in claim 9, furthercomprising an electrochemical reference electrode positioned within theinterior space of the shroud so as to be exposed to the fluid therein.19. A method of determining crack development of a monitored structurecomprising: (a) positioning a sensor assembly as in claim 9 in a fluidwhich contacts a component of interest associated with a monitoredstructure; (b) establishing an electrical connection between the sensorunit and the monitored structure so that the sensor unit and themonitored structure are at the same electrochemical potential; and (c)monitoring data signals generated by the sensor unit to determinepropensity for crack development within the component of interestassociated with the monitored structure.
 20. The method as in claim 19,wherein the monitored structure is a marine vessel.